The state of the art light source for integrated circuit lithography is the excimer laser, primarily the KrF 248 nm excimer laser. The laser provides illumination for a stepper or scanner machine which is used to produce the integrated circuits on silicon wafers. The stepper and scanner machines are very expensive costing several million dollars and they are typically operated virtually 24 hours per day seven days per week. Alignment of the beam delivery system from the laser to the stepper or scanner can be a complicated job. The alignment can be done with the laser operating using the UV beam for alignment. Such techniques are complicated by the fact that the UV beam is invisible and also presents a serious eye hazard. Also, sometimes alignment checks are needed when the laser is not operating. An alternative technique involves insertion of a small visible light laser somewhere in the optical train and using its beam to simulate the location of the UV laser beam.
FIG. 1 shows the principal features of a prior art 248 nm KrF narrow band excimer used extensively as an illumination source for integrated circuit lithography. The laser system comprises laser chamber 2 which contains a gain medium produced by high voltage pulses applied to two elongated electrodes discharging at rates of about 1,000 Hz through a circulating gas comprised of 0.1% fluorine, 1% krypton and the balance neon at about 3 atmospheres. The resonance chamber is defined by an output coupler 6 comprising a partially reflecting mirror (of about 0.1R) and a line narrowing module 7 comprised of: (1) a prism beam expander package 18 comprised of three prisms 8, 10 and 12, all mounted on a prism alignment plate 22 which can be moved laterally to precisely align the LNM 7 with output coupler 6 and (2) a pivotable tuning mirror 14 and a grating 16 configured in a Littrow configuration to reflect back through the prism beam expander to the gain medium, a selected narrow wavelength band of the spectrum produced in the gain medium. Alignment equipment in a prior art laser system includes a removable mirror and a positionable visible laser diode, mounted to output coupler module 6, on the side facing away from chamber 2. The mirror can be repeatably positioned to block the UV beam and fold the visible beam into the same optical path. The visible laser is provided with angle and position adjustments which allow the visible beam to be made collinear with the UV beam.
What is needed is a better, faster alignment system and technique for aligning narrow band UV lasers and associated stepper and scanner machines.